The use of the Doppler effect for detecting notion is well known. For example, in microwave intruder detection devices it is known to provide a diode oscillator mounted in a cavity resonator such as a simple wave guide tube. A diode mixer also mounted in the tube provides an output at the Doppler frequency. Such devices are very difficult to manufacture accurately, and thus in mass-production give rise to high rejection rates.
In GB1448266 there is described a microwave transmission-line circuit comprising a hybrid junction and four split-tee junctions, two of the split-tee junctions constituting power dividers and the other two constituting power combiners, wherein the inputs of the two dividers are respectively connected to two ports of the hybrid junction, wherein a first output of a first and a first output of the second of the dividers are respectively connected to a first input of a first and a first input of the second of the combiners, wherein the second outputs of the first and of the second dividers are respectively connected to the second inputs of the second and of the first combiners, and wherein the lengths of the interconnections of the split-tee junctions are such that the difference between the signal phase angles x and y appearing respectively at the output of the first and second combiners when respectively a signal is applied to the input of the first divider and when a signal is applied to the input of the second divider, is substantially an odd multiple of 90 degrees. It will be appreciated that the circuit described is relatively complicated, requires a relatively large number of components, and takes up a relatively large amount of space.
The oscillator and the aerial are each separately mounted, and, together with the necessary coupling means to the circuit, take up even more space.
In WO92/09905 there is described a motion detector unit for detecting motion by means of Doppler frequency shift which comprises a microwave circuit board, an antenna circuit board, and a ground plane layer, the two boards being superimposed with the ground plane layer intervening, the microwave circuit board comprising an oscillator and a mixer, and the antenna circuit board comprising transmit and receive antennae, each antenna having a respective feed stripline which substantially overlies an associated stripline on the microwave circuit board, the ground plane layer having, for each antenna, a respective slot to provide coupling between the microwave circuit and the antenna, the slot being resonant at the fundamental frequency of the oscillator, with said feed stripline and said associated stripline lying orthogonal to the resonant dimension of the slot and extending beyond the slot.
This construction provides many advantages over earlier designs, and the unit is quite suitable for mass-production techniques. Nevertheless, the unit still takes up a relatively large amount of space because of the need to provide at least two antennae on the antenna circuit board. This can be a disadvantage, particularly in certain intruder detection devices.